Process for producing A73A, a new Efrotomycin-like antibiotic in fermentation broth

ABSTRACT

There is disclosed a new antibiotic agent, A73A, produced by Streptomyces viridifaciens MA-4864 (ATCC 31495) under suitable conditions. Said antibiotic shows antibacterial and growth-permittant activity.

This is a division of application Ser. No. 051,318, filed June 22, 1979, now U.S. Pat. No. 4,262,002, issued Apr. 14, 1981.

SUMMARY OF THE INVENTION

This invention relates to a new antibiotic agent A73A with antibacterial activity, growth-permittant activity and to a method for its production. The antibiotic is produced by growing Streptomyces viridifaciens MA-4864 (ATCC 31495) in an aqueous nutrient medium comprising assimilable carbon and assimilable nitrogen sources under submerged aerobic conditions until substantial antibiotic is imparted to the medium.

The antibiotic is recovered from the fermentation medium by filtering off the mycelium. The filtrate is adsorbed and eluted on the appropriate resin. Further purification is effected by evaporation, extraction, precipitation and/or chromatography to yield substantially pure A73A.

A 73A is an antibiotic which is effective against gram-positive and selected gram-negative bacteria and may be used to treat infections in animals. Furthermore, A73A can be used as a growth-permitting agent for animals such as chickens and pigs.

DESCRIPTION OF THE INVENTION

This invention relates to the production by fermentation and isolation of a useful antibiotic substance. More particularly, this invention relates to the preparation of antibiotic A73A by cultivating Streptomyces viridifaciens under controlled conditions, followed by isolation of said antibiotic.

The antibiotic A73A is obtained by growing under controlled conditions the microorganism, Streptomyces viridifaciens, in a fermentation broth. The fermentation may be carried out in a media containing suspended nutrient matter or predominantly clear media wherein the media are substantially free of suspended nutrient media.

Based upon extensive taxonomic studies, Streptomyces viridifaciens was identified as an actinomycete and has been designated MA-4864 in the culture collection of MERCK & CO., Inc., Rahway, New Jersey. A culture thereof has been placed on permanent unrestricted deposit with the culture collection of the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Maryland 20852 and assigned accession No. ATCC 31495.

The classification keys for the genus Streptomyces and the culture descriptions of Streptomyces species found in Bergey's Manual of Determinative Bacteriology (8th Edition, 1974, Williams and Wilkins and International and Journal of Systematic Bacteriology, 18, 138 (1968) were searched for a Streptomyces species having morphological and cultural characteristics similar to those of MA-4864. In these aforementioned classical references, the Streptomyces viridifaciens shows morphological and close cultural characteristics with the A73A-producing culture MA-4864. Therefore, MA-4864 is designated Streptomyces viridifaciens.

the morphological and culture characteristics of Streptomyces viridificiens MA-4864 are set forth in the following table.

Morphology: Sporophores form branched structures with flexuous chains of spores often ending in hooks and open loops of 1 to 3 loose coils. Spores are oval to cylindrical, 1.2μ×1.7μ (970×). Spore surface as seen by electron microscopy is smooth. (V=vegetative growth; A=aerial mycelium; SP=soluble pigment).

Oatmeal agar (ISP Medium 3)

V: Reverse-reddish grayish brown

A: Powdery, gray (3 ig) mixed with white

SP: None

Czapek Dox agar (sucrose nitrate agar)

V: Colorless

A: Sparse, grayish white

Egg albumin agar

V: Reverse-grayish tan

A: Mixed gray and white

SP: None

Glycerol asparagine agar (ISP Medium 5)

V: Reverse-reddish tan

A: Powdery, mixed gray and white

SP: None

Inorganic salts-starch agar (ISP Medium 4)

V: Reverse-reddish grayish brown

A: Powdery, gray (3 ig) mixed with white

SP: None

Yeast extract-dextrose+salts agar

V: Reverse-reddish-tan

A: Mixed gray and white

SP: None

Yeast extract-malt extract agar (ISP Medium 2)

V: Reverse-reddish brown

A: Powdery, gray (3 ig) edged with white

SP: None

Peptone-iron-yeast extract agar

V: Tan

A: None

SP: None

Melanin: None

H₂ S production: None

Nutrient tyrosine agar

V: Tan

A: None

SP: slight browning of medium

Decomposition of tyrosine: Positive

Nutrient starch agar

V: Tan

A: None

SP: None

Hydrolysis of starch-good

Nutrient gelatin agar

V: Tan

A: None

SP: None

Liquefaction of gelatin-good

Skim milk agar

V: Tan

A: None

SP: Very light brown

Hydrolysis of casein: Positive

Litmus milk

V: Tan growing ring

A: None

Coagulation and/or peptonization: Peptonization, becoming alkaline

Carbon utilization

Pridham-Gottlieb basal medium+1% carbon source: +=growth; ±=growth poor or questionable; -=no growth as compared to negative control (no carbon source)

Glucose +

Arabinose +

Cellulose -

Fructose +

Inositol -

Lactose -

Maltose +

Mannitol -

Mannose ±

Raffinose -

Rhamnose -

Sucrose +

Xylose +

Temperature range (Yeast extract-dextrose+salts agar)

28° C.--Good growth with sporulation

37° C.--Moderate vegetative growth

50° C.--No growth

Oxygen requirement (Stab culture in yeast extract-dextrose+salts agar)

Aerobic

All readings taken after three weeks at 28° C. unless noted otherwise. pH of all media approximately neutral (6.8-7.2)

Color number designations taken from Color Harmony Manual, 1958, 4th Edition, Container Corporation of America, Chicago, Illinois.

In the case wherein the fermentation is carried out in media containing suspended nutrient matter the antibiotic is found mainly in the filtered broth. The antibiotic is isolated from the filtered broth by adsorbing the antibiotic on the appropriate resin and eluting.

In media containing no suspended nutrient matter, most of the A73A is found in the predominantly clear fermentation broth.

A preferred method for obtaining the antibiotic of this invention is by growing, under controlled conditions, the microorganism, Streptomyces viridifaciens MA-4864 in a medium containing suspended nutrient matter or in a clear medium substantially free of suspended nutrient matter and adsorbing the filtered broth on the appropriate resin and eluting. The eluate is concentrated. The concentrate is extracted by adjusting to acid pH and adding a water immiscible organic solvent. The solvent layer is drawn off and evaporated in vacuo. The residue is redissolved in an appropriate polar organic solvent such as ethyl acetate and added dropwise to nonpolar organic solvent such as hexane. The precipitate formed is chromatographed over silica gel to yield the antibiotic A73A.

In the process described above wherein extraction is carried out with water immiscible polar organic solvents, representative examples of said solvents include alkyl esters of lower alkanoic acids such as methyl formate, ethyl formate, methyl acetate, ethyl acetate, n-butyl acetate, isobutyl acetate, ethyl propionate; a ketone such as cyclohexanone; or a halogenated lower hydrocarbon such as chloroform, methylene chloride, carbon tetrachloride, ethylene dichloride, 1-chloro-2,2-dimethylpropane, tetrachloroethylene, or bromoform.

The antibiotic A73A isolated from the fermentation broth is subjected to purification by adsorption and chromatography over surface active adsorbing agents. A suitable surface active adsorbing agent is a hydrophobic non-ionic macro porous copolymer of polystyrene cross-linked with divinylbenzene known by the Rohm and Hoss trade names Amberlite XAD-1 to XAD-12. A preferred resin for purifying A73A is XAD-2. Suitable solvents are acetone and lower alkanols, e.g., aqueous solutions of methanol, ethanol, isopropanol, butanol and the like. The preferred solvent for eluting A73A from XAD-2 resin is 75% acetone:water. For further purification, silica gel is a suitable surface active adsorbing agent.

Streptomyces viridifaciens MA-4864 is simply illustrative of the type of strain of microorganism which can be used in the production of A73A and it should be understood that the present invention is not limited to organisms meeting these particular descriptions. This invention includes the use of the other microorganisms, including strains of actinomycetes either isolated from nature or obtained by mutation as, for example, those obtained by natural selection or those produced by mutating agents, for example, X-ray irradiation, ultraviolet irradiation, nitrogen mustards and the like which, under suitable conditions will yield A73A.

A73A is produced during the aerobic fermentation of suitable aqueous nutrient media under controlled conditions via the inoculation with the organism Streptomyces viridifaciens MA-4864. Aqueous media, such as those employed for the production of the other antibiotics are suitable for producing the antibiotic A73A. Such media contain sources of carbon, nitrogen and inorganic salts assimilable by the microorganism. The choice of media is not critical and the fermentation may be carried out in media containing suspended nutrient matter or predominantly clear media wherein the media is substantially free of suspended nutrient matter.

In general, carbohydrates such as sugars, for example dextrose, glucose, arabinose, maltose, raffinose, xylose, mannitol and the like and starches such as grains, for example, oats, rye, corn starch, potato and the like can be used either alone or in combination as sources of assimilable carbon in the nutrient medium. The exact quantity of the carbohydrate source or sources utilized in the medium depends in part upon the other ingredients of the medium but, in general, the amount of carbohydrate usually varies between about 1% and 6% by weight of the medium. These carbon sources can be used individually, or several such carbon sources may be combined in the medium. In general, many proteinaceous materials may be used as nitrogen sources in the fermentation process. Suitable nitrogen sources include, for example nutrient broth, yeast extract, yeast hydrolysates, primary yeast, soybean meal, cottonseed flour, hydrolysates of casein, corn steep liquor, distiller's solubles or tomato paste and the like. The sources of nitrogen, either alone or in combination, are used in amounts ranging from about 0.2% to 6% by weight of the aqueous medium.

Media described in the examples are merely illustrative of the wide variety of media which may be employed, and are not intended to be limitative.

The fermentation is carried out at temperatures ranging from about 20° C. to 37° C.; however, for optimum results it is preferable to conduct the fermentation at temperatures of from about 24° C. to 32° C. The pH of the nutrient media suitable for growing the Streptomyces viridifaciens MA-4864 culture and producing the antibiotic A73A should be in the range of from about 6.0 to 8.0.

A small scale fermentation of the antibiotic is conveniently carried out by inoculating a suitable nutrient medium with the antibiotic-producing culture and, after transfer to a production medium, permitting the fermentation to proceed at a constant temperature of about 28° C. on a shaker for several days. At the end of the incubation period, the fermentation broth can be filtered and the antibiotic adsorbed and eluted from the appropriate resin.

The small scale fermentation is conducted in a sterilized flask via a one-, two-, three-, or four-stage seed development. The nutrient medium for the seed stage may be any suitable combination of carbon and nitrogen sources. The seed flask is shaken in a constant temperature chamber at about 28° C. for a period of from one to two days and some of the resulting growth is used to inoculate either a second stage seed or the production medium. Intermediate stage seed flasks, when used, are developed in essentially the same manner; that is, part of the contents of the flask are used to inoculate the production medium. The inoculated flasks are shaken at a constant temperature for several days and at the end of the incubation period the antibiotic A73A is isolated as already described.

For large scale work, it is preferable to conduct the fermentation in suitable tanks provided with an agitator and a means of aerating the fermentation medium. According to this method, the nutrient medium is made up in the tank and sterilized by heating at temperatures of up to about 120° C. Upon cooling, the sterilized medium is inoculated with a previously grown seed of the producing culture, and the fermentation is permitted to proceed for a period of several days, as for example, from two to four days while agitating and/or aerating the nutrient medium and maintaining the temperature at about 28° C. The yield of A73A is generally between 20 mg. to 200 mg. per liter of production broth as determined by bioassay.

Physical Properties

Antibiotic A73A is purified by thin layer chromatography (CH₂ Cl₂, CH₃ OH, conc. NH₄ OH; 90:10:0.5) to give an amorphous slightly deliquescent yellow powder. Elemental analysis, calculated for C₄₄ H₆₂ N₂ O₁₀ : C, 67.84; H, 8.02; N, 3.60: found: C, 67.05; H, 8.11; N, 3.39. The mass spectrum of the trimethylsilyl derivative shows peaks up to m/e 1210 (M⁺) consistent with a hexatrimethylsilyl derivative of C₄₄ H₆₂ N₂ O₁₀, m.w. 778. The spectrum shows essentially the same fragmentation pattern as that of the closely related antibiotic A40A, filed in copending case U.S. Ser. No. 955,553, filed Oct. 27, 1978, now abandoned.

The structure of antibiotic A73A is the following: ##STR1##

In the above structure, the pyran has the following configuration at its asymmetric centers: S at the hemiketal carbon 2, S at the hydroxyl carbon 4, and S at the pentadienyl side chain bearing carbon 6.

Repeated thin layer chromatography in the above solvent system fractionates A73A into two closely running bands: R_(f) 0.25, A73A₁ and R_(f) 0.30, A73A₂. Both show field absorption mass spectra consistent with the above molecular weight. The infrared spectra of these are virtually the same but the ¹ H NMR spectra of the two show significant differences in the aliphatic regions. Base catalyzed interconversion of A₁ and A₂ can be demonstrated by NMR. The spectral data are interpreted in terms of interconversion of the stereochemistry at the ethyl bearing methine next to the amide carbonyl group. The data are shown in Table I for the partial structure in Formula 1. The spectra were obtained on CD₃ OD solutions and the chemical shifts are given in ppm downfield from tetramethylsilane with coupling constants given in parentheses. Antibiotic A40A differs from both A73A components in its stereochemistry at the hydroxyl bearing methine C-4. The ¹ H NMR data for the A40A partial structure are also shown in Table I for comparison. ##STR2##

                  TABLE I                                                          ______________________________________                                         A73A .sup.1 H NMR Comparisons for Formula 1                                    A73A.sub.1   A73A.sub.2    A40A                                                ______________________________________                                         a         4.53 d (6) 4.50 d (6)  4.13 d (6)                                    b         0.82 s     0.76 s      0.74 s                                        c         0.91 s     0.89 s      0.94 s                                        d         3.56 t (3) 3.56 t (3)  3.73 d,d (2.2, 11.5)                          e         1.92 d (3) 2.45 d,d (2.7, 14.5)                                                                       1.94 d d (2.2, 12.5)                          f                    1.66 (obscurred)                                                                           1.46 d,d (11.5, 12.5)                         g         2.37 t (7.5)                                                                              2.32 d,d (6.5, 8.5)                                                                        2.39 d,d (5, 10)                              ______________________________________                                    

Assay Procedure for Antibiotic Activity

Assays are run by the disc-plate procedure using 3/8 inch filter paper discs. The assay plates are prepared using Difco brain heart infusion agar, buffered at pH 6.0, at 10 ml. of agar per plate. A spore suspension of the assay organism, Bacillus circulans (MB 264) is diluted in sterile saline solution to a suspension having 45% transmittance at a wave length of 660 mμ. This suspension is added at 33.3 ml./liter of agar medium prior to pouring the plates.

The assay plates are held at 4° C. until used (5 day maximum). Following the application of the antibiotic-saturated assay discs the plates are incubated at 28° C. for a period of from 16 to 24 hours. Zones of inhibition are read as mm. diameter. They are used to determine relative potencies or when compared with a preferred reference standard, the potency in μg./ml. Assays of A73A in fermentation broths, mycelia and suspended nutrient matter separated from fermentation broths and in broths free of solids are performed after extracting the A73A into a suitable solvent. Assays on solutions containing A73A, 200 μg./ml. using 3/8 inch discs shows 16 mm. zones of inhibition. When such an assay is performed in a quantitative fashion, from 100 to 200 μg./ml. of antibiotic can be detected.

A73A shows activity against gram-positive bacteria, but only limited activity against gram-negative bacteria. In vitro, A73A is effective against Sarcina lutea (ATCC 9341), Streptococcus faecium (MB 2820) and Streptococcus agalactiae (MB 2875). Activity is also found against Vibrio percolans (ATCC 8461), Klebsiella pneumoniae (MB 1264), Xanthomonas vesicatoria (MB 815), Bacillus subtilis (ATCC 6633) and Alcaligenes faecalis (ATCC 213).

A73A is useful both as an antibiotic and as a growth-permittant agent in animals.

When A73A is used as an antibiotic, the specific means employed for administering it to the animal is critical, and only some of the methods now used or available for treating infected animals or animals susceptible to infection are satisfactory.

A73A can be used as an antibiotic, for example, in the form of pharmaceutical preparations which contain it in admixture or conjunction with an organic or inorganic, solid or liquid pharmaceutical excipient suitable for enteral, parenteral or local administration. Suitable excipients are substances that do not react with the antibiotic, for example, water, gelatin, lactose, starches, stearyl alcohol, magnesium stearate, talcum, vegetable oils, benzyl alcohols, gums, propyleneglycol, polyalkyleneglycols, white petroleum jelly, cholesterol or other known medicinal excipients. The pharmaceutical preparations may be, for example, tablets, dragees, ointments, creams or capsules, or in liquid form solutions, suspensions or emulsions. They may be sterilized and/or contain assistants, such as preserving, stabilizing, wetting, or emulsifying agents; solution promoters, salts for regulating the osmotic pressure or buffers.

Where it is desired to administer the antibiotic in dry, solid unit dosage form, capsules, boluses or tablets containing the desired amount of antibiotic are employed. These dosage forms are prepared by intimately and uniformly mixing the active ingredient with suitable finely divided diluents, fillers, disintegrating agents and/or binders such as starch, lactose, talc, magnesium stearate, vegetable gums and the like. Such unit dosage formulations may be varied widely with respect to their total weight and content of A73A depending upon factors such as the type of host animal to be treated, the severity and type of infection and the weight of the host. The antibiotic may be administered on a daily basis at from about 5 to 100 mg. per kilograms of body weight.

Included in this invention are the non-toxic, pharmaceutically acceptable salts of A73A, for example, the alkali and alkaline earth metal such as those derived from sodium, potassium, ammonium, and calcium or salts with organic bases, for example, triethylamine, N-ethylpiperidine, dibenzylethylenediamine.

In addition to its use as an antibiotic, A73A is useful as a feed additive to permit the growth of animals such as chickens, sheep and cattle. The use of A73A shortens the time required for bringing animals up to marketable weight.

When A73A is used as a growth permittant in animals, it can be administered as a component of the feed of the animals or may be suspended in the drinking water.

When A73A is used as a component of animal feed, it is first formulated as a feed supplement. In such feed supplements, A73A is present in relatively concentrated amounts intimately dispersed in an inert carrier or diluent. The feed supplement can be added to the feed or made into a premix by an intermediate dilution or blending step. By inert carrier is meant one that will not react with the antibiotic and one that may be administered safely to animals. Preferably, the carrier is one that is, or may be an ingredient of the animal ration. Typical carriers or diluents suitable for such compositions include, for example, distillers' dried grains, corn meal, citrus meal, fermentation residues, ground oyster shells, wheat shorts, molasses solubles, corn cob meal, edible bean mill feed, soya grits, crushed limestone and the like. The antibiotic is intimately dispersed throughout the carrier by methods such as grinding, stirring, milling or tumbling. Compositions containing from about 5 to 50% by weight of the antibiotic are particularly suitable as feed supplements.

Examples of typical feed supplements containing A73A dispersed in a solid carrier are:

    ______________________________________                                                             lbs.                                                       ______________________________________                                         (A)      A73A              5                                                            Wheat Standard Middling                                                                         95                                                   (B)      A73A             50                                                            Corn distiller's grains                                                                         50                                                   ______________________________________                                    

These and similar feed supplements are prepared by uniformly mixing the antibiotic with the carrier.

The feed supplement can be added directly to the feed or made into a premix by an intermediate dilution or blending step with an orally ingestible carrier. Compositions containing 0.03% to 5% by weight of the antibiotic are particularly suitable as premixes. These premixes are prepared by uniformly mixing the antibiotic with an orally ingestible carrier.

Such supplements or premixes are added to the animal feed in an amount to give the finished feed the concentration of A73A desired for growth permittant. In chickens, A73A is fed at a final concentration of between 10-100 ppm per ton of feed in order to achieve the desired growth permittant result. In the case of swine, including swine infected with Mycoplasma hyorhinis (PPLO) A73A may be administered in the feed at similar levels.

In the above discussion of this invention, emphasis has been placed on solid compositions wherein the A73A is mixed with an edible carrier in a feed supplement, in a so-called premix or in the final feedstuff. This is the preferred method of administering the A73A. An alternate method is to suspend the A73A in the drinking water of the animals. The quantity that may be suspended in the water without undue settling is limited. Emulsifiers or surface-active agents may be employed for this latter purpose.

It will likewise be understood by those skilled in this art that special feed supplement formulations and finished animal feeds containing A73A may also include vitamins, other antibiotics and growth-permitting agents and other nutritional substances.

A73A is useful against poultry PPLO at a range of 5 to 100 mg./kg. A preferred range for a single dose is from 35 to 45 mg./kg. For reasons of convenience, a preferred method of administering the antibiotic in the treatment of PPLO is to admix the A73A with the animal feed. A preferred range for PPLO is from 0.0055% to 0.02% by weight of feed.

The examples which follow illustrate methods by which the product of this invention may be obtained. The claimed process is capable of wide variation and modification and, therefore, any minor departure therefrom or extension thereof is considered as being within the skill of the artisan and as falling within the scope of this invention.

EXAMPLE 1 Shake Flask Fermentation

A portion of the slant culture containing MA-4864 having the following composition:

    ______________________________________                                         Peptone              5.0    g.                                                 Glucose              10.0   g.                                                 Meat extract         5.0    g.                                                 NaCl                 3.0    g.                                                 Agar                 15.0   g.                                                 Distilled water      1000   ml.                                                pH 7.0                                                                         ______________________________________                                    

is used to inoculate a baffled, 250-ml. Erlenmeyer flask containing 50 ml. of KW seed medium having the following composition:

    ______________________________________                                         Medium KW                                                                      ______________________________________                                         Dextrose              20.0   g.                                                Bacto Peptone         5.0    g.                                                Difco Meat Extract    5.0    g.                                                NaCl                  5.0    g.                                                Primary Yeast         3.0    g.                                                Distilled water       1000   ml.                                               pH 7.0                                                                         then CaCO.sub.3       3.0    g.                                                ______________________________________                                    

The above flask is shaken at 28° C. on a 220 rpm shaker (2-inch throw) for three days. The growth from this seed flask is used to inoculate three separate 250-ml. Erlenmeyer flasks each containing 40 ml. of Medium KH, JH, and JT media, respectively, using 2 ml./flask of (5%) inoculum.

    ______________________________________                                         Medium KH                                                                      ______________________________________                                         Tomato Paste           20.0   g.                                               Primary Yeast          10.0   g.                                               CPC Industrial Starch                                                          Mod.                   20.0   g.                                               CoCl.sub.2 . 6H.sub.2 O                                                                               5.0    g.                                               Distilled water        1000   ml.                                              pH 7.2-7.4 with NaOH                                                           ______________________________________                                    

    ______________________________________                                         Medium JH                                                                      ______________________________________                                         Corn Meal             20.0   g.                                                Distilled Solubles    10.0   g.                                                Soybean Meal          15.0   g.                                                Na Citrate            4.0    g.                                                CaCl.sub.2 . 2H.sub.2 O                                                                              0.5    g.                                                MgSO.sub.4 . 7H.sub.2 O                                                                              0.1    g.                                                CoCl.sub.2 . 6H.sub.2 O                                                                              0.01   g.                                                FeSO.sub.4 . 7H.sub.2 O                                                                              0.01   g.                                                Polyglycol P-2000     2.5    ml.                                               Distilled water       1000   ml.                                               pH 6.5                                                                         ______________________________________                                    

    ______________________________________                                         Medium JT                                                                      ______________________________________                                         Tomato Paste          20.0   g.                                                Primary Yeast         5.0    g.                                                Glycerol              20.0   g.                                                NaCl                  2.5    g.                                                Distilled water       1000   ml.                                               pH 7.2 with NaOH                                                               ______________________________________                                    

These flasks were shaken at 28° C. on a 220 rpm shaker (2-inch throw) for 96 hours and submitted for assay. The assays showed in vitro antibacterial activity, against E. coli and B. subtilis.

EXAMPLE 2 Procedure for Handling Lyophiles of MA-4864

A lyophilized tube of S. viridifaciens is treated in the following way, in order to prepare additional lyophiles or grow into an inoculum for large scale production.

1. The pellet from the lyophilized tube is suspended in 1.0 ml of 0.85% saline.

2. 0.2 ml of this suspension is used to inoculate each of our slants of the following medium:

    ______________________________________                                         Ardamine (autolysed yeast)                                                                         10.0 gm                                                    Glucose             10.0 gm                                                    MgSO.sub.4 . 7H.sub.2 O                                                                             .05 gm                                                    Agar                20.0 gm                                                    Distilled Water     1 liter                                                    pH adjusted to 6.5-6.8                                                         ______________________________________                                    

3. The slants are incubated at 28° C. until well sporulated.

4. The growth from the four slants is suspended in 5 ml of 15% sterile skim milk solution prepared from BBL's Skim Milk Powder.

5.0.15 ml aliquots of the suspension are placed in freeze-dry ampoules and freeze-dried.

EXAMPLE 3 Production of Antibiotic A73A

The contents of a lyophile, prepared as in Example 2, Step 4, is used to inoculate a 250-ml baffled Erlenmeyer flask containing 50 ml of Medium KE. The flask is incubated at 28° C. for 48 hours while being shaken on a 220 rpm rotary shaker.

After 24 hours, a 10-ml. sample from the above flask is added to a 2-liter baffled Erlenmeyer flask containing 500 ml. of Medium KE. The 2-liter flask is incubated at 28° C. for 72 hours on a 150 rpm rotary shaker.

After 72 hours, 500 ml. of the broth from the 2-liter flask is added to a 189-liter stainless steel fermentor containing 160 liters of Medium KE having the following composition.

    ______________________________________                                         Medium KE                                                                      ______________________________________                                         Dextrose (Cerelose)     160    gm                                              Starch Modified CPC     1600   gm                                              Meat Extract            480    gm                                              Ardamine PH             800    gm                                              NZ Amine Type E         800    gm                                              Magnesium sulfate - MgSO.sub.4 . 7H.sub.2 O                                                            8      gm                                              Potassium phosphate, Monobasic                                                 KH.sub.2 PO.sub.4       29.1   gm                                              Sodium phosphate, Dibasic                                                      Na.sub.2 HPO.sub.4      30.4   gm                                              Volume                  160    liters                                          pH to 7.0-7.2 using NaOH                                                       Add Calcium carbonate - CaCO.sub.3                                                                     80     gm                                              ______________________________________                                    

The stainless steel fermentor is operated at a temperature of 28° C. using an air flow of 3 cubic feet per minute (cfm) and an agitation rate of 150 rpm for a period of 48 hours. The physiology of the broth is as follows.

    ______________________________________                                         Age in Hours     0        24       48                                          pH               7.2      6.7      7.7                                         ______________________________________                                    

After 24 hours, 43 liters from the above stainless steel fermentor is added to a 756-liter stainless steel fermentor containing 467 liters of Medium KH having the following composition.

    ______________________________________                                         Medium KH                                                                      ______________________________________                                         Tomato paste            20.6   lbs                                             Primary yeast           10.3   lbs                                             Modified Starch CPC     20.6   lbs                                             Cobalt chloride - CoCl.sub.2 . 6H.sub.2 O                                                              2.3    gm                                              Polyglycol 2000         150    ml                                              Volume                  467    liters                                          pH 7.2-7.4 using NaOH                                                          ______________________________________                                    

The fermentor is operated at a temperature of 28° C. using an air flow of 10 cfm and an agitation rate of 130 rpm for a period of 96 hours. To the fermentor is added Polyglycol 2000 not exceeding 0.1%. The physiology profile is as follows.

    ______________________________________                                         Age In Hours                                                                             0      12    24   36  48   60  72   84   96                          pH        6.9    6.8   6.7  6.7 6.8  6.9 7.0  6.8 7.35                         ______________________________________                                    

Whole broth (16 liters) is adjusted from pH 6.7 to pH 9.0 at room temperature. The whole broth is filtered through a Buchner funnel containing a super-cel precoat. The filtered broth is adsorbed onto 1 liter of XAD-2 using a 20-minute contact time (the dimensions of the column are 4 cm. ID×80 cm. long). The column is then washed with one column volume deionized water. Activity is eluted with 2.0 liters of 75% acetone:25% deionized water mixture. The eluate is concentrated to 625 ml. in vacuo at 30° C. The pH is adjusted from pH 7.2 to 4.0 and extracted two times with 625 ml. of ethyl acetate. The extracts are combined and evaporated in vacuo at 30° C. to an oily residue. The residue is redissolved in 100 ml. of ethyl acetate, and this solution is added dropwise to 500 ml. of hexane with vigorous stirring. The yellow precipitate formed is chromatographed on preparative TLC plates (silica gel) to yield the pure A73A. 

What is claimed is:
 1. A method of producing A73A having the following structure: ##STR3## wherein the pyran has the following configuration at its asymmetric centers: S at the hemiketal carbon 2, R at the hydroxyl bearing carbon 4, and S at the pentadienyl side chain bearing carbon 6 which comprises cultivating an A73A-producing strain of Streptomyces viridifaciens in a fermentation broth containing assimilable sources of carbohydrates, nitrogen and inorganic salts under aerobic conditions until a substantial amount of A73A is produced in the fermentation broth and recovering said antibiotic.
 2. The method of claim 1 wherein the Streptomyces viridifaciens is designated ATCC
 31495. 3. The method of claim 1 wherein the fermentation broth is filtered and the filtered broth is extracted with a water immiscible polar organic solvent.
 4. The method of claim 3 wherein the extraction solvent is selected from the group consisting of methyl formate, ethyl formate, methyl acetate, ethyl acetate, N-butyl acetate, isobutyl acetate, ethyl proprionate, methyl ethyl ketone, cyclohexanone, chloroform, methylene chloride, carbon tetrachloride, ethylene dichloride, 1-chloro-2,2-dimethylpropane, tetrachloroethylene, bromoform, N-butanol, and methyl isobutyl ketone. 